The present invention relates to a process for the production of L-malic acid (LMA) by biotechnical conversion of fumaric acid. During this process, a reaction mixture is obtained containing, besides the desired LMA, unreacted fumaric acid and other nonionic compounds from the biotechnical conversion. LMA and fumaric acid are normally present in the form of salts.
The biotechnical conversion of fumaric acid into LMA is known and is carried out by means of various processes. It is possible to use bacteria (DAS 2,363,285), immobilized bacteria (Europ. J. Appl. Microbiol. 3 . 169-183 [1976]; U.S. Pat. No. 3,922,195), fungi (DOS 3,310,849), immobilized fungi (DOS 2,450,137), or the enzyme fumarase isolated from microorganisms (DAS 2,415,310). The LMA salt can be present in the reaction mixture in a moderately high concentration or in a high concentration (DOS 3,310,849; U.S. Pat. No. 592,977 of Mar. 23, 1984 and DOS 3,434,880; U.S. 654,070 of Sept. 24, 1984). Furthermore, the biotechnical conversion of glucose into LMA has been described by a symbiosis between a fungus and a bacterium (J. Ferment. Technol. 54 : 197-204 [1976]).
Various processes have been known for the separation of LMA from the reaction mixture. For example, a large amount of a calcium compound can be added to the fermentation batch whereby the thus-formed LMA precipitates as the calcium malate as early as during the fermentation (DOS 1,417,033). The reaction mixture can also be treated with cation exchangers; the eluates, after concentration, are filtered and evaporated (DOS 3,247,981). Furthermore, it is possible to separate, by acidification, the unreacted fumaric acid from the mixture and thereafter precipitate the LMA as the calcium salt. This salt can be reacted with sulfuric acid to form calcium sulfate and LMA, the latter being treated with an ion exchanger to remove the dissolved calcium sulfate.
The conventional processes for obtaining LMA by biotechnical conversion of fumaric acid have the following characteristics, inter alia:
Upon acidification of the reaction mixture for the purpose of separating the unreacted fumaric acid, a salt is formed from the added acid and the cations contained in the reaction mixture; this salt, after the subsequent calcium precipitation of the LMA, though remaining in the filtrate for the largest part, is also found to a certain extent in the filter cake of Ca malate.
The filter cake must be washed salt-free and clean. During this step, losses of Ca malate occur whereby the LMA yield is reduced. The filtrate and the washing water are discarded.
The alkaline solution added prior to the biotechnical conversion for neutralizing the fumaric acid is practically unrecoverable, and thus is entirely lost after a one-time use.
During reaction of Ca malate with sulfuric acid, Ca sulfate (gypsum) is obtained as an unavoidable by-product. The removal of this product burdens the process with additional expenses.
It is frequently difficult to obtain the LMA in the desired high purity with a simultaneously high yield.
A problem, therefore, exists in improving separation of LMA from the reaction mixture so that LMA can be obtained in high purity with a large yield, under economical conditions.